XML 50 R50.htm IDEA: XBRL DOCUMENT v2.4.1.9
Property, plant and equipment (Details 3) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, plant and equipment [Line Items]      
Interest capitalized $ 8,586us-gaap_InterestCostsCapitalized $ 6,033us-gaap_InterestCostsCapitalized $ 8,659us-gaap_InterestCostsCapitalized
AFUDC - borrowed 3,022us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest 2,767us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest 2,483us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest
AFUDC - equity 5,803us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity 3,322us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity 4,530us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity
Property, plant and equipment 9,693,171us-gaap_PropertyPlantAndEquipmentGross 8,803,866us-gaap_PropertyPlantAndEquipmentGross  
Less accumulated depreciation, depletion and amortization 4,166,407us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment 3,872,487us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment 3,608,912us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Net property, plant and equipment 5,526,764us-gaap_PropertyPlantAndEquipmentNet 4,931,379us-gaap_PropertyPlantAndEquipmentNet 4,498,839us-gaap_PropertyPlantAndEquipmentNet
BOE average rate for amortization under units-of-production method (in dollars per BOE) 21.17us-gaap_AmortizationExpensePerPhysicalUnitOfProduction 17.41us-gaap_AmortizationExpensePerPhysicalUnitOfProduction 15.28us-gaap_AmortizationExpensePerPhysicalUnitOfProduction
Oil and natural gas properties not subject to amortization 132,141us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative 124,869us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative 191,794us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative
Electric:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 1,457,101us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
[1] 1,315,822us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
[1] 1,150,584us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
[1]
Natural gas distribution:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 1,904,759us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
[1] 1,776,901us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
[1] 1,689,950us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
[1]
Pipeline and energy services:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 1,220,233us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
962,172us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
816,533us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
Exploration and production:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 3,402,879us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
3,060,848us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
2,764,560us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
Construction materials and contracting:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 1,529,942us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
1,510,355us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
1,504,981us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
Construction services:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 144,395us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
134,948us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
130,624us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
Other:      
Property, plant and equipment [Line Items]      
Property, plant and equipment 50,937us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
49,997us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
50,519us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
Regulated: | Electric: | Generation      
Property, plant and equipment [Line Items]      
Property, plant and equipment 627,952us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
570,394us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
 
Weighted average depreciable life in years 42 years    
Regulated: | Electric: | Distribution      
Property, plant and equipment [Line Items]      
Property, plant and equipment 343,692us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricDistributionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
308,202us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricDistributionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
 
Weighted average depreciable life in years 39 years    
Regulated: | Electric: | Transmission      
Property, plant and equipment [Line Items]      
Property, plant and equipment 229,997us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricTransmissionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
196,824us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricTransmissionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
 
Weighted average depreciable life in years 48 years    
Regulated: | Electric: | Construction in progress      
Property, plant and equipment [Line Items]      
Property, plant and equipment 150,445us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
141,365us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
 
Regulated: | Electric: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 105,015us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
99,037us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ElectricMember
 
Weighted average depreciable life in years 15 years    
Regulated: | Natural gas distribution: | Distribution      
Property, plant and equipment [Line Items]      
Property, plant and equipment 1,481,390us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasDistributionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
1,384,587us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasDistributionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
 
Weighted average depreciable life in years 40 years    
Regulated: | Natural gas distribution: | Construction in progress      
Property, plant and equipment [Line Items]      
Property, plant and equipment 59,310us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
46,763us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
 
Regulated: | Natural gas distribution: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 364,059us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
345,551us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
 
Weighted average depreciable life in years 27 years    
Regulated: | Pipeline and energy services: | Transmission      
Property, plant and equipment [Line Items]      
Property, plant and equipment 449,276us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasTransmissionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
418,594us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasTransmissionMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 53 years    
Regulated: | Pipeline and energy services: | Gathering      
Property, plant and equipment [Line Items]      
Property, plant and equipment 39,595us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasGatheringAndProcessingEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
39,597us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_GasGatheringAndProcessingEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 20 years    
Regulated: | Pipeline and energy services: | Storage      
Property, plant and equipment [Line Items]      
Property, plant and equipment 43,994us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_GasStorageMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
42,939us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_GasStorageMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 60 years    
Regulated: | Pipeline and energy services: | Construction in progress      
Property, plant and equipment [Line Items]      
Property, plant and equipment 5,386us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
6,937us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Regulated: | Pipeline and energy services: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 39,910us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
39,504us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_RegulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 33 years    
Nonregulated: | Eliminations      
Property, plant and equipment [Line Items]      
Property, plant and equipment (17,075)us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConsolidationEliminationsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
(7,177)us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConsolidationEliminationsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
 
Nonregulated: | Pipeline and energy services: | Midstream      
Property, plant and equipment [Line Items]      
Property, plant and equipment 227,598us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MidstreamMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
213,063us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MidstreamMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 16 years    
Nonregulated: | Pipeline and energy services: | Construction in progress      
Property, plant and equipment [Line Items]      
Property, plant and equipment 314,304us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
188,641us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Nonregulated: | Pipeline and energy services: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 100,170us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
12,897us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
 
Weighted average depreciable life in years 18 years    
Nonregulated: | Exploration and production: | Oil and natural gas properties      
Property, plant and equipment [Line Items]      
Property, plant and equipment 3,337,177us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_OilAndNaturalGasPropertiesMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
[2] 3,017,879us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_OilAndNaturalGasPropertiesMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
[2]  
Nonregulated: | Exploration and production: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 65,702us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
42,969us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ExplorationAndProductionMember
 
Weighted average depreciable life in years 8 years    
Nonregulated: | Construction materials and contracting: | Land      
Property, plant and equipment [Line Items]      
Property, plant and equipment 125,372us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
125,551us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
 
Nonregulated: | Construction materials and contracting: | Buildings and improvements      
Property, plant and equipment [Line Items]      
Property, plant and equipment 70,566us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingAndBuildingImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
70,000us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingAndBuildingImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
 
Weighted average depreciable life in years 19 years    
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment      
Property, plant and equipment [Line Items]      
Property, plant and equipment 921,564us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MachineryVehiclesAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
906,774us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MachineryVehiclesAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
 
Weighted average depreciable life in years 12 years    
Nonregulated: | Construction materials and contracting: | Construction in progress      
Property, plant and equipment [Line Items]      
Property, plant and equipment 8,709us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
13,315us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
 
Nonregulated: | Construction materials and contracting: | Aggregate reserves      
Property, plant and equipment [Line Items]      
Property, plant and equipment 403,731us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_AggregateReservesMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
[3] 394,715us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_AggregateReservesMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
[3]  
Nonregulated: | Construction services: | Land      
Property, plant and equipment [Line Items]      
Property, plant and equipment 5,265us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
4,821us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
 
Nonregulated: | Construction services: | Buildings and improvements      
Property, plant and equipment [Line Items]      
Property, plant and equipment 17,936us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingAndBuildingImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
16,628us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingAndBuildingImprovementsMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
 
Weighted average depreciable life in years 20 years    
Nonregulated: | Construction services: | Machinery, vehicles and equipment      
Property, plant and equipment [Line Items]      
Property, plant and equipment 112,973us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MachineryVehiclesAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
105,991us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= mdu_MachineryVehiclesAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
 
Weighted average depreciable life in years 6 years    
Nonregulated: | Construction services: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment 8,221us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
7,508us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
 
Weighted average depreciable life in years 4 years    
Nonregulated: | Other: | Land      
Property, plant and equipment [Line Items]      
Property, plant and equipment 2,837us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
2,837us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
 
Nonregulated: | Other: | Other      
Property, plant and equipment [Line Items]      
Property, plant and equipment $ 48,100us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
$ 47,160us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
/ us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis
= us-gaap_UnregulatedOperationMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
 
Weighted average depreciable life in years 23 years    
[1] Includes allocations of common utility property.
[2] Amortized on the units-of-production method based on total proved reserves at a BOE average rate of $21.17, $17.41 and $15.28 for the years ended December 31, 2014, 2013 and 2012, respectively. Includes oil and natural gas properties accounted for under the full-cost method, of which $132.1 million and $124.9 million were excluded from amortization at December 31, 2014 and 2013, respectively.
[3] Depleted on the units-of-production method.